Due to the sharp increase in the price of oil, attention has been focused in recent years on developing alternative energy sources, such as solar energy. The sun provides the earth with ample energy, but problems have been encountered in converting the sun's photo energy into other useful forms of energy, such as electrical energy or storable chemical energy.
One device which has been utilized to convert solar energy into electrical energy is the photovoltaic cell which employs semiconductors absorbing electromagnetic radiation in the visible and near IR region of the sun's spectrum. For a semiconductor to absorb light, the light must be at least of some minimum energy, referred to as the bandgap of the semiconductor and being characteristic of the particular semiconductor and its crystalline structure. Thus, all light of an energy greater than or equal to the bandgap is available for absorption and energy conversion, whereas all light of an energy less than the bandgap is not. The bandgap also defines the maximum potential output voltage of the semiconductor when used in an energy conversion device. It is therefore preferable that the bandgap of the semiconductor be relatively small so that a high percentage of the solar spectrum will be available for absorption, but not so small that the output voltage of the cell will be too low to be useful. The peak of the solar spectrum occurs about 2.5 eV with the curve dropping sharply as energy increases and tapering off slowly as energy decreases. Based on this, the most potentially efficient semicondutors for photovoltaic cells have bandgaps of approximately 1.5 eV.
One of the major problems of solar energy as a power source and photovoltaic cells as a conversion device is the inconsistency of sunlight, which is not available for power generation at night or on cloudy days. Photovoltaic cells only produce electricity and only when the sun is shining. Electricity may be converted to chemical energy and stored in batteries, but the batteries are bulky and expensive. Therefore, it is desirable to have an energy conversion device that is capable of converting solar energy to a storable form of energy so that energy will still be available during no or low sunlight periods. The photoelectrolysis cell is such an energy conversion device. Sunlight is used as the power source to drive an endothermic chemical reaction involving two redox couples, such as the photoelectrolysis of water to produce hydrogen and oxygen. The photoelectrolysis products may either be stored, or fed directly into fuel cells or other similar devices where the chemical energy is converted to electrical or mechanical energy. When a semiconductor is to be used as a photoelectrode in a photoelectrolysis cell, the electrolysis potential of the electrolyte and the overpotentials of the electrodes must also be taken into account when determining the optimum handgap for the semiconductor. Considering these factors, the optimum bandgap for a semiconductor in a photoelectrolysis cell is between 1.75eV and 2.5 eV. For a more detailed discussion of the theoretical principals of photoelectrolysis cells see Deb and Wallace, and Noufi and Warren, Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 248, pp 38-57, and 80-87 (1981).
Previous materials for use as photoelectrolysis electrodes have primarily consisted of metal oxides or compound semiconductors. The compound semiconductors have desirable bandgaps, but tend to degrade when used as photoelectrodes in photoelectrolysis cells. The metal oxides. conversely, are generally stable but have bandgaps on the order of 3.0 eV or greater and thus are less efficient because a large portion of the solar spectrum is unavailable for absorption.
Thus, there is need in the art for semiconductors with small bandgaps on the order of 1.75 eV to 2.5 eV which are stable when used as photoelectrodes in photoelectrolysis cells.
Accordingly, it is an object of the present invention to provide a photoelectrochemical cell employing a class of efficient photoelectrodes which do not degrade when used in photoelectrolysis cells.
it is another object of the present invention to provide a photoelectrochemical cell employing a class of photoelectrodes with bandgaps on the order of 1.75 eV to 2.5 eV.
A further object of the present invention is provide a class of efficient and stable photoelectrodes which are made from relatively inexpensive starting materials.